The present invention relates to a fabrication method and structure of a Printed Circuit Board Assembly (PCB Assembly, hereinafter PCBA), and a tool for assembly thereof.
Due to the demand for small-aspect, light and powerful electronic products, it is necessary for a design rule of a semiconductor chip to lay out denser wiring and more devices in a limited area thereof, resulting in increased density and decreased pitch of terminals on the semiconductor chip and/or package thereof. When attaching a BGA (ball grid array) package with terminal pitch of 0.5 mm or less, for example, on a circuit board, alignment therebetween presents extreme difficulty. Although more alignment deviations are acceptable when assembling the BGA package resulting from self-alignment capability of solder balls, acting as terminals, of the BGA package, the acceptable deviation decreases with ball pitch of the BGA package. When ball pitch of the BGA package is 0.5 mm or less, unacceptable alignment deviation between the BGA package and circuit board becomes more often, resulting in solder joint opening and/or short when reflowing the solder balls, thus requires reworking or scraping the circuit board, negatively affecting process yield and cost. Even if solder joints experience neither open nor short, the quality thereof may be negatively affected, resulting in a defect such as decreased joint area, making it impossible for solder joints to form ideal lantern shape. The farther the solder joints from the center of the BGA package, the more the alignment deviation, results in more deterioration of the joint quality. External stress on the outer solder joints increase during shipping, warehousing, and use of the assembled electronic device, accelerating fatigue on the outer solder joints, negatively affecting the reliability of the assembled electronic device.
FIG. 1A is a top view of a PCB 100. The PCB 100 has a solder mask 105 on a surface. The solder mask 105 has a plurality of openings 102 with a width W and being arranged at a pitch P, wherein W is less than P and the pitch is the distance between geometric centers of two neighboring openings 102. A plurality of inner pads 103 and a plurality of outer pads 104 are exposed from the openings 102. The outer pads 104 are arranged beyond the inner pads 103. As shown in FIG. 1A, two lines of solder pads 104 sandwich one line of solder pads 103. The pads 103 and 104 can be SMD (solder mask defined) pads, NSMD (non-solder mask defined) pads, SMD-NSMD combinations thereof, or other types of pads. A closed dash line 106 indicates the predetermined position for attaching an electronic device to the PCB 100 subsequently.
FIG. 1B shows a top view of a conventional tool 150 for applying the solder paste over the PCB 100 using stencil printing method. The tool 150 is a metal plate having a plurality of openings 151 arranged corresponding to pads 103 and 104 of PCB 100. When forming solder paste over the PCB 100, solder volume overlying each pads 103 and 104 may vary. As a result, the volumes of solder joints formed by reflowing the solder paste with the solder balls of the BGA package are different.
Referring to FIG. 2A, the volumes of adjacent solder joints 62 for some pads 103 and 104 are oversized, which may result in solder bridge defects. FIG. 2B shows another possible defect. The outer solder joints 62 are undersized, thus increases the possibility of unsoldering between pads 12 of electronic device 10 and pads 104 of PCB 100, resulting in failure of electrical connection between pads 12 and pads 104. Even if electrical connection is formed between pads 12 and pads 104, the actual connecting area therebetween may be too small to provide sufficient mechanical strength and may affect product reliability.